SECTION I. 

 Foods. 



In comparing the metamorphosis of matter in animal and vegetable 

 organisms it has been found that in both cases there exists a certain 

 relationship between such changes and the surrounding media. In the two 

 classes of organisms, however, these processes are diametrically oppo- 

 site ; for, while we found that the higher plants remove for nutritive pur- 

 poses C0 2 from the atmosphere and returned O to it, the animal economy 

 retains a portion of the oxygen of the atmosphere, not remaining fixed 

 as such in the body, but to be again returned to the atmosphere as 

 C0 2 and H 2 0. For plants, consequently, since we found that the carbon 

 of the C0 2 and a portion of the hydrogen of the HjO become fixed in 

 their tissues, the atmosphere is a true food ; for animals it merely en- 

 ables tissue metabolism to take place, and permits of the maintenance 

 of animal heat. The development and growth of plants is dependent on 

 the liberation of oxygen and the appropriation of the inorganic con- 

 stituents of their foods. In animals life depends upon the constant 

 appropriation of oxygen, its union with the different constituents of the 

 body, and final elimination through the lungs and skin as C0 2 and H 2 0, 

 and through the bowels and kidneys in other simple^compounds. There- 

 fore, through the absorption of oxygen there is produced no increase in 

 bulk of the animal body, but rather a decrease. To meet this waste 

 of the organism there must be a constant appropriation of tissue-con- 

 stituents. Such substances are called foods. 



Nutrition may be defined as the functions which are concerned in 

 the preservation of the individual. Foods may, therefore, be defined as 

 any substances which may serve nutritive purposes. The body being in 

 a constant state of mutation, the constituents of the organism are little 

 by little eliminated as the result of this mutation, and to preserve the 

 necessary balance must be replaced. There must, therefore, be an exact 

 correlation between the constituents of an organism and the aliments 

 required by that organism. The demand for aliment is governed by .the 

 waste ; if the supply is not as great as the waste, the body loses weight. 

 If, on the contrary, as in youth, the supply is greater than the waste, 

 the body increases in weight. When the losses of the economy reach 

 a certain degree without a sufficient reparation having taken place, 

 when the disassimilation exceeds the assimilation, the sense of hunger 



(157) 



